Electrical timing circuit



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w ,uned 2.25.8@ s! L3M@ 1 /N VE N 7` ORS A 7' TORNE Y Patented Mar. 6, v19455 ELECTRICAL 'ruime cmcurr Wilton T. Rea, Bayside, and Jeii'erson E. Wil-y kerson, Brooklyn, N. Y., assignors to Beil Telephone Laboratories, Incorporated, vNew York, N. Y., a corporation of New York Application February 28, 1942, Serial No. 432,823

17 Claims.

This invention relates to electrical timing circuits and, more particularly, to timing' circuits used in start-stop telegraph systems for timing of operations therein.

Itis an object of the invention to provide tim- (cl. 17a-ss) ing circuits of this kind which are more stable A Different types of timing systems have been proposed in the past for this purpose.

Thus, it has been general practice to use a rotary start-stop distributor for determining the timed intervals'. `This type of device has certain disadvantages, when operated at high speed and where high accuracy of timing is desired, because themechanical starting mechanism as well as the constantly running friction clutch used with this apparatus are subject to wear and, therefore, tend to variably affect the operation of the distributar, particularly at starting. Another disadvantage of this type of device, when used at high speed, is to be found in the fact, that there is a certain minimum length of the distributor segments below which dei'lnite, well-formed timing impulses cannot be produced by the segments; this has been found to be particularly disadvantageous in certain testing systems for checking the operating conditions oi the teletypewriter plant.

It has also been proposed to use a timing system without mechanical rotating parts but based on oscillating relay circuits or on oscillatingv circuits using gas-filled tubes for the timing of the individual pulses and using similar oscillating relay or tube circuits, more slow acting, for stopping the pulse timing oscillator at the end of each signal train.

, Reference may be had to United States Patents 1,771,445, issued to Parker-Vernam on July 29, 1930, and 2,154,623,` issued to Kinkead on April 18, 1939, disclosing oscillating relay circuits of this kind, and 2,133,456, issued to Kinkead on `October 18, 1938, disclosing oscillating circuits using gas-iilled tubes for timing purposes. Reference .may also be had to'United States Patent 2,132,678, issued to Cowan October 11, 1938, disclosing a signal distortion measuring 'device for teletypewriter circuits which uses a rotating startsto distributor for timing the operations or the measuring device.

With the steady demand for increased eml ciencyl ofthe telegraph plant, the speed of telegraphoperation has been greatly increased during the last few years land further increase in speed may be anticipated. It is therefore evident that a higher degree of accuracy of operation of commercial telegraph apparatus will be necessary. For purposes of checking the operations of such telegraph apparatus, testing systems of different types have beenl developed which operate on a start-stop -basis and such testing systems must necessarily be accurate within very narrow limits, in order to be able to indicate small deviations from the ideal operation of the'commercial apparatus. 'l It is therefore a-'principal object of the invention to provide a start-stop timing system which is highly accurate and dependable in operation,

and which will require less maintenance than other harmonics thanthebasic.

present timing systems using rotary distributors.

It is a more specific object to provide a testing system of the kind disclosed in the patent to Cowan, referred to above, but adapted for operation with a Vtiming system of the type provided in accordance with the invention for the purpose of. securing a -higher degree of accuracy andof enabling the testing system to measure a higher percentage of distortion. A

In accordance with a feature of the invention, an oscillatory circuit is provided which is adapted to produce oscillations of voltage having a true sinusoidal wavel shape and capable of being started from the steady-statecondition substantially without extraneous transients, so that the .first cycle will be substantially identical with the succeeding cycles.. vThe circuit is also capable of being quickly damped to the steady-state condition within one cycle. The oscillatory circuit is of'simpliiied design to enable it to produce a sinusoidal wave shape and associated with it is a vacuum tube circuit'adjusted to'y have a constant amplication factor within-the'operating range and to sustain the `oscillations at constant kamplitude' throughout at least one signal series ofk pulses. During the stop condition the circuit` is held cocked by a start circuitincluding a sttable direct current source, whereby the oscillating energy is stored inductively while the static energy remains practically zero. The opening of this start circuit 'starts the oscillatorycircuit upon its rst sinusoidal cycle'v without transients or Other objects and features of the invention will be apparent from the following detailed description of a specific testing system embodying the invention. It should. however, be understood that the various features of the invention may be incorporated in telegraph systems other than that described hereinafter, which is disclosed by way of example, for the purpose of fully explaining the invention and its mode of operation as incorporated in a practical system.

ln the following detailed description `of the testing system and its operation reference will be made to the attached drawings. in which:

Figs. 1, 2 and 3, when properly placed with respect to cach other in accordance with the diagram in Fig. 6, show the circuit arrangement;

Fig. 4 shows an alternative impulse producing circuit which may be substituted for the impulse producing circuit included in the system shown in Figs. 1, 2 and 3; and

Fig. 5 is a showing the relative timing of various operating conditions; in the circuit.

As already stated above, the system in various respects is similar to that disclosed in the Cowan patent and is adapted to be used for the same purposes, namely, to determine the appearance of normal and abnormal conditions in start-stop telegraph circuits as they aect the start-stop signals in such circuit.

Reference may, therefore, be: had to the Cow patent for an explanation of the diierent types of distortion that may be observed in start-stop signals by means of a testing system of the present type. The observations are made on two meters,

as in the Cowan system, namely, the peak-voltmeter which gives steady indications of the maximum total distortion of any individual pulse, and the bias-meter which gives integrated indications of bias present in a plurality of successive pulses and which may be switched to indicate displacement of mark-to-space or space-to-mark transitions or average displacement of both.

Referring now to Figs. 1, 2 and 3 of the drawings, the system comprises a number of main portions as follows:

The "input circuit," shown in Fig. 1, whereby the set is connected, for example, to a commercial operating circuit and whereby the incoming signals are applied to the testing equipment.

The comparison circuit, shown-in Fig. l, in which the incoming signals are received and caused to set up charges on apair of unit condensers which, in turn, aiect the two meters in accordance with their acquired charges once for each unit pulse of a series of pulses representing a character. '1111s circuit includes the bias-meter.

An "impulse circuit, shown in Fig. 2, for estabiishing a series of uniformly separated reference instants, one for each unit pulse in a signal series. This includes the "condenser timing circuit" which conditior the unit condensers for each test.

A character timing circuit," shown in Fig. 2, for stopping the impulse circuit at the end of a character seri.

A start-stop relay circuit, shown in Fig. 1, for starting the timing operations in response to an incoming start pulse and for restoring the system to normal in response to the stopping action of the character timing circuit.

The "peak-voltmeter circuit shown in Fig. 3, and connected to the comparison circuit for giving an indication of the maximum distortion in the incoming signals during a convenient time interval, and obtaining its power through suitable equipment from an alternating current source.

A power supply circuit," shown in Fig. 3, for supplying constant current and various potentials to the test system and obtaining its power through suitable equipment from an alternating currentsource.

The system, as shown in the drawings, is arranged for operation under a number of diierent conditions and for this reason includes a number of manual keys and neutral-type relays for adapting the different portions of the circuit to the specic conditions existing in the commercial circuit which is to be tested. Thus, the circuit, as shown in the drawings, may be adapted to test signals using a 'Z1/2 unit code or an 8% unit code and it is apparent that it may be further adapted for a rl-unit code or other start-stop codes. The circuit, as shown, may also be adapted for difierent signaling speeds at the different codes. Thus, signals may be accurately tested having dot frequencies of '15, 23, or 28 cycles per second corresponding, for example, to 40, and 'l5 words per minute for a code using 11,5 units. The circuit is also adapted to test either neutral-type signals or polar signals.

A distortion measuring set of this type is usually permanently installed in a switchboard in the telegraph exchange and in that case is made available for tests at the various operators positions by means of extension circuits and test set appearances each including the nessary jacks, meters, patching cords, indicator lamps and keys; the appearances are interconnected to cooperate in showing busy and idle conditions and normal or abnormal conditions of the test set at the various operators' positions. A circuit arrangement for such interconnection of operators positions for the use of a common distortion measuring set at any one position is disclosed in United States Patent 2,244,566, issued June 3, 1941, to J. T. N eiswinter, and it will be readily apparent to those skilled in vthe art how the input .circuit shown in Fig. 1 of the present drawings may be f incorporated in such an interconnecting system.

It will first be assumed that the measuring set is to be used for the observation of start-stop signals -in a commercial circuit using the 'I5/2 unit code at a dot frequency of about 23 cycles per second and using signals of the neutral type, with marking pulses represented by current condition and spacing pulses represented by no-current condition in the line. Each character will thus be represented by a series of -pulses beginning with a spacing start pulse and ending with a marking stop pulse and having ve selecting pulse intervals between the start and stop pulses. Thus, a

signal series of pulses representing a character begins with a start transition from marking to spacing followed by transitions I, II, III, IV and V from marking to spacing and spacing to' marking at transition points l, 2, 3, l and 5, respectively, in accordance with the code, and ending with a stop" transition from spacingto marking. In a commercial system the length o! a stop pulse is 1.42 times the length of the unit pulse during continuous transmission. An example of such a signal series representing a character is shown diagrammatically in Fig. 5-A.

The signals impressed upon the test circuit may be those passing between stations on a busy commercial circuit or they may be signals specially transmitted over an'idle commercial circuit. 'Ihe trafiic need not be interrupted or otherwise disturbed by the tests.

i a stop interval.

- Y j i 2,870,055

Maut circuit i A The incoming lineeirciiii;A lui, which may lbe connected in any desirable'manner to a commercial circuit; is :connected by the insertion of plug |02 intojack |03 to the line circuit within the' set; thiscircuit is normally `closed by normal conktacts of relay |04."A When the plug |02 is inserted the closing'of contacts 03a of the Jack causes the operation'of relay I 04 which, in operating, extends the'line circuit to contacts oi' relay |05, where the line circuit remains closed through normal contacts of that relay.

A busy lamp |06 is now lighted over a circuit from battery through the lamp, make contact c of relay |04, conductor |-|3, break contacts of key 201 and break contacts of relay to a ground applied from the interconnecting circuit, referred toabove, for the operators positions in a manner of no importance for an understanding of the invention.

' Relay |05 is next operated from battery at make contacts e of relay |04 and thereby extends the incoming'line circuit 'to the upper winding of polar relay |40. Relays |04 and |05 remain operated until the plug |02 is removed from the jack |03 at the end of an observation.

l Comparison circuit Relay has a lower biasing winding normally tending to operate the relay to its spacing When, as `ust described, the line cir-- .break contact c of relay |05 through the upper winding of relay |40 and over break contact d to ground for holding relay |40 normally to marking-as shown in vthedrawings; when relay. |05 is operated, thisholding circuit is opened and relay |40 is held in marking condition by the marking condition of the incoming line circuit. Under this condition relays 'and |60 are held to marking and relays |40, |50 and |60 are in con-- dition to respond to the transitions of incoming pulses, relays |50 and |60 operating shortly after relay. |40 tov their markingy and spacing contacts.

-With the circuit in the condition shown, unit condensers |5| and |6| are connected over the marking contactsof relays |50. and simulr taneously .to two different circuits. respectively, and these connections are reversed when relays |50 and |60 are operated to spacing, Condenser |6| isfnormally connected to the constant current supply circuit over marking contactof relay |60, constant current conductor 336a, resistance 210,

constant current'conductorz336, meter 335to the' constantcurrentxsupplycircuit at the plate e yof vacuum tube 330. -As will be explained subsequently, the constant current supply circuit is arranged lto deliver a substantially constant current into a varyingvimpedance or. voltage, such as that `of a charging'or discharging condenser, so that 'normally thecondenser Ill will become ycharged over the traced circuit with aconstant current irrespectively of its increasing potential. However,

' under the stop condition, when condenser |6| is potentiometer 32|.

also connected over the marking contact of relay "land conductor 322to adjustable point 32 I` of The potential applied from' point 32| to condenser |6| remains xed and is called the .stop-compensating voltage.

The other unit condenser' |5| is normally connected over the marking contact of relay |50 to a reference potential, which circuit may be traced over the marking contact ofrelay |50, over resistance A|52 and condenser |53 rconnected in multiple by contact e of relay I |f8, over contact d of relay H8, conductor 342a, resistance 34|, con- .tacts of key 343, primary winding of transformer 344, conductor'342 to point 348 of a potentiometer circuit from negative battery through resistance 341, the-cathode plate circuit of vacuum tube 340 to ground. As will be explained'subsequently, the reference potential at point 348 is maintained at a standard value which is used for comparison purposes in the tests to be described. Potentiometer 320 is so adjustedl that the stopcompensating voltage at point 32| and the reference potential of point 348 will be equal during the stop condition. Thus, both unit Acondensers |5I and |6|V are charged during the stop interval to a potential equal to the reference potential. Ihe circuits for bias-meter |90 will be described below,

Peak-voltmeter circuit The left winding of transformer 344 has a grounded mid-point and is connected through rectifier vacuum tubes 35| and 352 for rectication of current in both directions and for charging of condenser 36| over the circuit to ground at potentiometer 362. Under the rest condition of the system, as shown in the drawings, no current flows in the transformer and condenser 36| remains uncharged; however, during test conditions when current ilows in one or the other kdirection through the primary of transformer 344, condenser 36| will acquire a, charge which is applied to the grid of vacuumv tube 360 for the peakvoltmeter 310. This meter is connected to the conjugate points of a Wheatstone bridge having the following arms: the first arm including resistance 363, cathode-plate circuit of tube 360, resistance 364; seccnd arm, right side of potentiorneter 365; third arm, left side of potentiometer 365; fourth arm, resistance 366. Direct current is applied to the supply points of the bridge circuit from the terminals 395 and 396 of a power supply equipment. With no charge on'condenser 36| and a corresponding plate currentin tube 360,

' the potentiometer 365A is adjusted to give zero current in the meter 310. 'A'Ihe meter will then give /an indication each time current 'flows through transformer 344 and causes condenser v36| `toacquire a higher charge, and the deflection of the meter vwill be iii-,proportion to the peak current inthe primary lwinding of the Y transformer 344.

meter may be calibrated. The bridge circuit'for f the meter is supplied with power from a source of alternating current 390, which may be a commercial power plant, .through transformer 39| and full-wave rectiiier392 to the terminals 395 and 090, with series inductance 303 and shunt condenser 304 provided to smooth the ripple in the output circuit.

Start-stop relay circuit During the stop condition the impulse circuit and the character timing circuit are held inoperative by meansof relay |50. Polar relays |10 and l |80 have lower biasingv windings which are nor- `mally energized, the biasing winding of relay |10 normally operating the relay'to marking, and that of relay tending to operate'the relay to spacing. 'Ihe circuit for the upper operating marking. When relay |40, in response to a spacing pulse, operates to its spacing contact, negative potential is applied from potentiometer |42 to the plate circuit of tube 220 rendering the tube 220 non-conducting; relay |80 then operates to spacing, thereby starting the character timing circuit and the impulse circuit upon a cycle of operations.

Character timing circuit The character timing circuit includes a set of timing condensers 222, 222a and 222b of which the'rlrst two normally are connected in multiple for use under the assumed conditions, namely, for 'a speed of 60 words per minute. Condensers 222 and 222a in multiple are normally charged from positive battery of 130 volts through resistance 22| to ground through th`e rectifier vacuum tube 225. Under this condition the upper plate of condenser 222 and the right-hand plate of condenser 222a, connected to point 228, will be` at plus potential and the other plates, connected to point 221, will be at ground potential with a charge potential of 130 volts. Resistance 22| is comparatively low for quick chargingof thecondensers during the stop interval. With plus potential connected to the plate of rectiiler 225, this tube will be in conducting condition; with the same potential applied to the grid and y with 'positive potential applied to the plate of tube 220, this tube'will be in conducting condition.

When the relay |00 operates to spacing in response tothe operation of relay |40 to spacing, a comparatively low negative potential on the potentiometer |0| is applied over the spacing contact of relay |00 through the operating winding of relay to the upper point 220 at one side or condensers 222 and 222a. With a potential of volts applied in this manner to the point 220, the potential of point 221 willbe -150 volts and both tubes 220 and 225 will be rendered nonconductive. Under this condition, condensers 222 and 222a will slowly discharge over a circuit from positive 130 volt battery through high resistance 223 to the negative potential on potentiometer |0|. This discharge condition will prevail until he charge on the condensers has been reversed nd raisedv to a value at which the condenser potential is about +20 volts, at which time the potential of point 221 will pass through zero and the tubes 220 and 225 will again become conductive. With tube 220 in conducting condition and relay |40 in marking position, relay |00 will be operated back to its marking position, thereby removing the negative potential from the condensere at point 220, so that the condensers thereafter will be quickly charged from plus battery over resistance 22| and through the rectiiier 5 tube 225, thereby restoring the character timing circuit` to its original condition. The timing oi' the charging period for these condensers is such that they will be fully charged to 130 volts within 1 a fraction of the stop interval and the timing l0 of the .discharging period of these condensers is 15 and 225 conductive, shortly after the normal arrival of the "stop transition. The charging cycle for condensers 222 and 222a is shown diagram, matically in Fig. 5D, and the corresponding variation in the cathode-grid potential inA tube 2o '220 is shown in Fig. 5-E.

It will be noted that a very high Vdischarging potential is used in this circuit and that the ef` fective point at which the timing period is terminated is only slightly past the middle point of 25 the condenser'voltage curve. Thus, the battery potential is 130 volts and the condenser potential is 130-.|520 or 150 volts at the moment the dis charge commences, the total beingv 280 volts, which insures a steep voltage characteristic. The

3o charge reverses at zero volts on the condenser and the current continues until the condenser has a voltage of. 20 volts in the opposite direction.

Therefore, the timing period. will be terminated at a point at which the rate of change of Yvoltage with time is high. The time at which the condenser reaches a charge corresponding to 20 volts in reverse direction thus will be sharply delimited and high accuracy of timing is thus secured. This circuit has been found to operate satislfactorily with signals of the 7l/2 unit code at a speed of 23 cycles per second with the following circuit constants:

0 in the csciuatcry circuit and the left-hand portion 240 operating as a detector in the output circuit of the oscillatory circuit.

The' oscillatory circuit comprises the two-winding inductance coil 23| and the variable condenser 232, the lower junction point being by the break contacts d of relay 202, but may be replaced by condenser 232a or by both the condensers 232a and 232|: by operation of relays 202 and 204 for changing the frequency of oscillation, as will be further explained below.

The cathode-anode circuit 'of tube 230 includes a direct current source and resistance 231 and is connected across the lower windingof coil 23| for compensation of the decrement of the oscillatory circuit. The control electrode is connected over the break contacts b of relay 202 to the adjustable point of potentiometer 233 which is bridged across the upper winding of coll 23| over break contact c of relay 202; by proper adjustment of the potentiometer point the oscillatory circuit may have zero decrement and the amplitude of oscillation will remain substantially constant over a period of more than ten oscillations for a given adjustment. The potentiometer 233 may be replaced by the potentiometer 233a by operation of relay 202 when the circuit is to be used at another frequency, thereby obviating readjustment of the tube circuit eachtime the set is used at different operating frequencies in the line circuit.

The condenser 232 may be adjusted for oscillation at 45.6 cycles per second corresponding to an operating speed of 22.8 cycles per second or 60 words per minute `for a 71/2 unit code. Thus the circuit may be adjusted to produce exactly one cycle for each standard pulse period of the signals, thereby establishing a series of accurate comparison instants in the test set.

However, for start-stop operation it is essential that the comparison instants be definitely related to the start instant during successive incoming pulse series and be uniformly spaced during each pulse series; it is therefore, necessary that the oscillatory circuit be started and stopped for each pulse series. Thus during the stop condition the pulse circuit is held cocked, that is, it is prevented from oscillating by the application of a negative potential from the potentiometer 8| over the marking contact of relay |80, conductor |82, resistance 236, break Contact a of relay 202 and resistance 234. to the upper junction of the oscillatory circuit. A steady-state condition is thus established during which current ows through the windings of coil 23 I storing the energy of oscillation in the magnetic flux of the coil, and during which the charge on the condenser 232 is practically zero.

Upon removal ofthe negative potential by th operation of relay |00 in response .to an incoming start transition the oscillatory circuit will be free to oscillate and the iirst'oscillation will be 'identical with subsequent oscillations and thus ing effect of resistances 234 and 236. With the proper values of the negative .potential at potentiometer |8I andof the resistances 234 and 236 for a given frequency. a current'will flow in coil 23| during stop condition lwhich will establish a desired amplitude of oscillation, at the same time as the effect of resistances 234 and 236 will be such as to return the circuit to the steady state in a critically dampedmanner, and well within the normal stop period.

than those that determine the regular waveshape of the oscillations. The importance of producing a sinusoidal wave shape, therefore, is apparent. Thus when by design the circuit is incapable of producing other harmonics than the basic, the oscillations may be started without transients and will remain constant if adjusted for zero decrement.

The main requirements for producing a sinusoidal Wave form are, that the circuit must be simple and must contain no other energy storing elements than the inductance coil and the condenser. Stated more specifically the require-- ments are:

4There must be no other inductance than the Inasmuch as it is important to accurately establish the ilrst reference instant, it is essential that the rst cycle be started ol! without delay or distortion, as .by transients or harmonics other windings of the coil in the oscillatory circuit; The two windings of the coil should have a coeflicient of coupling equal to unity; The vacuum tube should be operated within the straight-line portion of its characteristic; and The amplification factor of the tube should bel such that dissipation of energy is compensated for.

A circuit designed to answer these requirements may be held cocked by a current through coil 23| from the damping circuit equal to the oscillatory current at maximum amplitude. When the current supply is disconnectedtby relay the current in the coil will decay, following a cosine curve identical with that of the subsequent oscillations; at the. same time the condenser voltage will rise from its steady-state value of about zero volts, following a sine curve identicalwith that of the subsequent oscillations. Since the circuit by design is incapable of producing harmonics other than the basic, the rst cycle will, be free of extraneous transients. y.

For the purposes of the invention the desired sinusoidal operation of this circuit may be secured with sufcient accuracy bythe use of apparatus and connections of the general high quality required for commercial equipment of this general type. Whereas they amplification constant may-be calculated from the other ,circuit constants it is attainable in practice by adjustment of the tube circuit until the decrement disappears for a desired number of oscillations.

Thus the oscillating circuit will produce a uniform series of oscillations corresponding to each pulse series of the incoming signals and will beA ready for a new series of oscillations every time a start pulse arrives. A series of voltage oscilla- `tions produced by the oscillatory circuit during a character series of pulses is shown diagrammatically in Fig. 5F.

The following ciircuit constants 'for the impulse circuit have been found tc give satisfactory operation at 22.8 cycles per second:

Vacuum tube 230-240 made by Radio v Corporation ofv Ameriea-- v;k.,-v 6F8G An alternative arrangement for an oscillatory circuit having operating characteristics and main design principles similar to those of the oscillatory ciircuit shown in Fig. 2 and describedabove, is shown in Fig. 4.

Corresponding parts in the two circuits have been similarly numbered so that the circuit of Fig. 4 may readily be substituted for the corresponding circuit in Fig. 2.

Briefly describing the circuit in Fig. 4, the oscillatory circuit comprises the upper winding of the two-winding inductance 23|' and the variable condenser 232', the lower junction point being grounded and the upper junction point being connected to the output circuit.

The cathode-anode circuit of tube 230' includes a direct current source and the lower winding of coil 23|' for feeding back to the oscillatory circuit under control of the control electrode or` grid which is connected to the adjustable point f of potentiometer 233 bridged across the oscillatory winding of coil 23|'.

The circuit may be adjusted to perform exactly like that in Fig. 2 though by diil'erent inherent operations, and lt may be cocked in the same manner by the application of a steady-state current from the potentiometer |81' over conductor |82' and resistance 234' and through the oscillatory winding of coil 23i'.

The requirements listed above for the circuit in Fig. 2 also hold for this circuit, whichv thus will include the main features of producing a sinusoidal wave shape with substantially zero change of amplitude for at leastvtwenty oscillations, and of starting after being cocked without extraneous transients, its performance being illustrated in Fig. 5 1?. This circuit thus is adapted to serve the Purposes of the invention.

The detector tube 24| has its cathode-plate circuit connected over the primary winding of transformer 24|, break contact a of relay 2|3, potentiometers 242 and 243 and break contact c of relay 2|3. The potentiometers 242 and 243 are connected acro plus the minus batteries in series over break contacts b of relay 202 in such a manner that a positive potential may be supplied from potentiometer 2421 to the plate and a lowerpotential from potentiometer 243 to the iilament of tube 24|. i By adjustment of the potentiometer points individually, a desiredpotential diiference may be applied to the cathodeanode circuit in the tube and by mechanically locking the two contact points together for simultaneous adjustment c! them this potential dierence may be maintained at the same .time as a proper potential relation with respect to the grid of the tube is established for orienting purposes. It should be understood that the two potentiometers may be replaced by a single potentiometer with two adjustable points.

The control grid of tube 24| is connectedlt'o the upper junction of the oscillatory circuit so that an oscillating voltage will be impressed thereon. For a given adjustment of potentimeters 242 and 243, the tube 24| will be non-conducting while the potential impressed upon the grid is below a predetermined value. slightly negative relative to the cathode potential, and will be conducting when the potential on the grid is above this value. Thus, the tube will be conducting during the upper portions of each cycle of the oscillations, as indicated by the shaded portions in Fig. 5-F above the horizontal line f-L which represents the critical potential at which the condition in the tube changes. The corresponding current pulses through the detector tube 240 and the primary winding of transformer 24l are shown.diagram-` matically in Fig. 5-G. By proper adjustment of the potentimeters 2I2 and 243 the critical potential and thus line f-f-may be varied between the positive and negative maximum amplitudes of the oscillations. thereby shifting the instant at which the detector tube 24U becomes non-conducting between the instants or the maximum and minimum amplitudes.

In this manner, two voltage impulses will be generated in the secondary winding of transformer .2M during each cycle of the oscillations, namely, an impulse of relatively low maximum amplitude in a negative direction at the instant the detector tube 240 becomes conducting on the rising slope and an impulse of relatively high maximum amplitude in a positive direction at the instant'the tube 24U becomes non-conductive on the descending slope. These impulses in the secondary winding of transformer 24| are schematically shown inFig. 5-H.. The positive voltage pulses from the secondary of transformer 24| are impressed upon the grid of the gas-filled tube 250 to render this tube conducting, whereas the negative pulses will have no elect upon the tube 253. 'I'here will thus be an impulse impressed upon the tube 250 once for each cycle and the impulse may be accurately oriented withrespect to the start transition by means of the potentiometers 242 and 243 and is usually oriented to occur slightlybefore the center of each standard pulse period, not as established in the line circuit under test, but as established by the slightly delayed operations of relays |53 and itil, for control The condenser timing circuit, including the gas- 40 oued tubes 25s and zal, is provided w substantially instantaneously discharge one or 'the other of the unit condensers I5! and lil, whichever happens to be connected to the constant current circuit 33E-33Go at the middle of each pulse period after relay i1! has been operated to spacing at the arrival of a s transition.

The circuits for the gas-filled tube 25| may be traced as follows. The cathode is connected over conductor 253, spacing contact of relay 11| to the negative constant current conductor' 33611 to which is also connected, say. the grounded unit condenser li. 'I'he plate circuit is connected to ground through lnductance 255 and resistance 251. The control grid is connected over resistance 252, the secondary winding of transformer 24| directly to the negative constant current conductor 336. Thus, with no'voltage impulse present in the secondary of transformer 24|, the grid of tube 250 receives a negative potential relative to the cathode whichis equal to the potential drop in resistance 21|! of l5 volts due to the constant cur- 'rent iiowing into unit condenser lil and under this condition the gas-filled tube remains nonconducting. However, when a positive voltage impulse of, say, volts from transformer 24| is impressed on the grid of tube 25|, the tube becomes ionized and thus establishes an oscillatory discharging circuit from the grounded negatively charged condenser lil, through the tube and the inductance 256 and resistance 251 to ground. The condenser now quickly discharges and, due to the inductance 25S, tends to charge in the positive direction.

As will appear from the detailed description below of the operation of the system the magnitude of the negative voltage on condenser |6| taken as an example, at the time it begins to discharge through the tube 250, depends on the arrival time of the last preceding transition. If, therefore, the

v condenser current were permitted to flow through the tube to charge the condenser in the positive direction, the tube would not be extinguished and the recharging of the condenser could not begin until the instant the current attained zero value;

the positive condenser potential at this instant being nearly equal to the initial negative potential, it too would depend on the arrival time of the last transition. However, since the recharging of the condenser to negative potential is utilized for comparison purposes relating to the arrival time of the next transition, it must be made independent of any previous transition. This is attained by means of the circuit including the gasfllled tube 260.

The circuit for the gas-filled tube 260 may 'be traced as follows. y The plate is connected through resistance 261 to positive battery and is also connected through a normally charged condenser 255 to the plate circuit of tube 250 and the oscillatory circuit just described. The cathode is connected to ground. The control grid is connected through resistance 265 to the constant current conductor 336e and thus normally receives a negative potential, thereby maintaining the tube 260 non-conducting. During the oscillatory discharge of condenser I=6| through inductance 256, described above, the negative potential on the grid of tube 260 will quickly decrease with that of the constant current conductor 336a; when this potential passes through zero the tube 260 is red, thereby lowering the positive potential of the lower side of condenser 25'5 to the sustaining plate potential of tube 260 and thus forcing the upper side of condenser 255 and the plate of tube 250 sufficiently negative'to extinguish the latter tube. This thus removes the discharge condition from condenser ISI, which now begins to charge to negative potential from the constant current conductor 336e. The firing of tube 260 establishes an oscillatory circuit comprising condenser 255 and inductance 256, and-when the current in this circuit becomes zero, tube 260 is extinguished.

By this arrangement the current through con? denser |6| is interrupted by the tube 250 at the instant the condenser potential reaches zero. The time interval between maximum and zero amplitude of potential in the oscillatory circuit being fixed by the frequency vof oscillation and thus being independent of the maximum amplitude, the discharging time will be constant and the condenser will begin to recharge to'negative from zero charge and at a predetermined instant, and thus independently of the previous transition. 'Ihe charge and discharge of the unit condensers and |6| under the control partly by the condenser timing circuit and partly by the comparison circuit is illustrated in Figs. 5-1 and 5-J, respectively.

It will be noted that, when relay |16 opens its spacing contact at the beginning of the stop pe riod, the cathode of tube 250 will be disconnected from the constant current conductor 336a. However, a permanent circuit is established from the this type. If the condensers were discharged in' a resistive circuit, as would be the case with a. mechanical distributor o`r contact interrupter, the current wave would be of exponential form, in which the ratio of |peak value of current to total charge dissipated is very high. Thus the discharge yof the unit condensers inthe present system may be completed in a much shorter tim than in the prior art systems.

The condenser timing circuit has been found to operate satisfactorily with ythe following circuit constants:

Gas-filled tubes 250 and -260 manufactured by Western Electric Company Type 338A Condenser 25| microfarad .001 Resistance 252 ohms 225,000 Condenser 255 microfarads.- 4 Inductance 256 henries 18 Resistance 25'| ohms-- 40 Condenser 262 microfarad-- .0002 Resistance 265 ohms 400,000 Resistance 210 do A1,000 Condenser 21| microfarads-- 4 VResistance 212 ohms-; 100,000

The potential drop over resistance 210 is about 30 volts, and is normally applied in negative direction to the grid of tube 250. The impulses from transformer 24| reach about 60 Volts applid in positive direction to tube 250, firing the With the condenser i 5| or |6| of 5.4 microfarads in Vcircuit, with inductance v256 and re. sistance 251, the time for bringing the, charge down through zero will be about .002 second, and the peak current throughtube 250 and inductance 256 will not exceed .6`ampere assuming an initial condenser voltage of 165. With the circuit constants listed above the discharge of the unit condensers will be completedl withinten per cent of the dot period, -making the unit condensers available for test purposes ,during ninety per centof the marking or spacing interval, respectively, and thereby permitting accurate measurements of distortions close to plus vand minus forty-five per cent. 'I l Power supply circuit The power supply circuit'derives power yfrom any convenient source of'alternating current A3H) which may be a commercial power supply system. I'he current from' thesource r3|0 passes through a transformer 3H vand a full-wave rectifier tube 3|2'to the supply terminals 3|5v and 3|6. An inductance 3|'3`is serially included'and con-` denser 3|4 is connected across this circuit to smooth the ripples in the output.'

The reference voltage supply circuit includes the resista-nce 341 and the ,cathode-plate ,circuit of vacuum tube '340 forming a potentiometer which establishes the reference voltage .at point 348. Under normal conditions' the current consumption of this circuit is low. However, when a load is connected to" conductor-342, variation of the potentiaiof point 348 wiu be nmited by the action of tube 340. Whenfor example, the potential of point tive, the grid-cathode potential becomes less neg- 348 tends to become more nega ative. This in turn causes an increased now of plate current, thereby causing the potential of point 3&8 to become less negative and thus compensating for the original tendency of this point. This action takes place at the instant one of the unit condensers I5I, |6| is connected to point 348, for comparison purposes, charged to a higher negative potential than the reference potential; when the condenser potential is less negative y than the reference potential the tube action will be reversed. This arrangement, therefore, maintains a substantially constant reference voltage at the times the comparisons between the reference voltage and the voltage of condenser i5! and |61 are made. Since the rate of change ci potential drop versus current in the anode-cathode circuit of tube 340 is very low, this circuit may be looked upon as a battery of very low resistance which permits the unit condenser to be quicklydischarged with a heavy current, which in turn will give a large deflection of the peakvoltmeter, as will be vfurther explained below. Condenser 349 serves to further decrease the impedance of the reference voltage source.

A resistance bridge comprising resistances 32u, 325 and 329 is connected across the output terminals SI5 and 3|6, and a portion of the potentiometer 32|) supplies the stop-compensating voltage over variable contact 32|, as already referred to. The potentiometer 32E supplies an adjustable potentialto the control grid b of the vacuum tube 340 for the reference potential ciraaneen f of potential in its output circuit, its voltage tends to rise excessively when its output circuit 336 is` constant current conductor 336e.

cuit 342. In this manner the current in tube 34d may be controlled automatically to insure that the reference potential is equal to the stop-compensating voltage during the stop condition re-V gardless of normal variations of the power supply voltage.

As alreadyexplained, the constant current is y passed through potentiometer 320 only when relay I'Il isin marking position, as during the stop interval. There are, therefore, two current conditions in potentiometer 32|! which thus tend tcreact upon the potential of point 323 in the resistance bridge. This would consequently also aifectthe reference. potential, which depends on the grid potential in tube 340 derived from potentiometer 325. To practically eliminate these variations in potentials, and' also to reduce the eifect of power voltage variations, a. voltage regu- The stop-compensating voltage and the refer- Vence potential may be checked on the meter 383 which is normally connected through key 38| tomeasure the stop-compensating voltage on conductor 322 and which, in the alternate position of key 33|, will be connected to the point 34| of the reference potential, replacing resistance 382 in this circuit to insure that the reference potential remain unaffected by operation of key 33|.

The constant current circuit being designed to supply a constant current against a wide range The voltage-regulating devices connected to different points in the circuit to stabilize operationsare of well-known construction and are adapted to serve this purpose by their specic voltage-current characteristic. They are gasfilled tubes with low current consumption at; normal voltage. but they consume a quickly rising current as the impressed voltage rises above the normal value. Devices of this type are manuiactured by the Radio Corporation of America, Incorporated.

The regulator tube 266 is of type VRl05-30 and is connected to conductor 336g to limit the potential o f this conductor to about volts.

The regulator tube 324 isof type VR-30 and is connected across potentiometer 328 to stabilize the stop-compensating voltage.

The two regulator tubes 321 are of the type VR105-30 and are serially connected acrossthe cathode and the screen grid of tube 33B to stabilize the constant current.

Normal operation transition I from spacing t'o marking and arriv- 'a ing ahead of the standard transition time indicated by the vertical line l; transition 1I from marking to spacing arriving late with respect to the standard transition instant 2; transition III from spacing to marking and arriving late with respect to the standard transition instant 3; no

transition IV; transition V from marking to spacing concurring with the standard transition instant 5 and the "stop transition from spacing to marking concurring with the standard transition instant 8.

.In response to the start" transition relay I" operates to spacing, thereby operating relays Ill and |33 to spacing.

Relay Ill removes the unit condenser lli from the reference potential conductor 2a and connects it to the constant current conductor 3301:. As shown in Fig. 5 1, the condenser begins to charge beyond the reference potential, indicated by the horizontal line i'i. Relay |60 removes condenser IBI from the stop-compensating voltage, applied to constant current conductor' 333e over the marking contact of relay |1|I during the ,stop condition, and connects it to the reference potential. These operations in response to a start" transition do not affect the bias meter |90 or the peak-voltmeter 310.

Relay |40, in operating to spacing, applies negative potential through potentiometer |42 and upper winding of relayY |80 to the plate circuit of the tube 220 which, therefore, becomes currentless. Relay |80 operates to spacing by means of its lower biasing winding. In spacing position relay I" operates relay I III in a circuit from -20 volts on potentiometer Isl through upper winding of relay |10, conductor |22. resistance 22| to plus battery. Tube 225 becomes non-conducting and the character timing circuit is thereby conditioned for the vslow discharge o f condensers 222 and 222a through high resistance 222.l A

negative, slowly rising potential is thus applied I to the grid of tube 220 as illustrated in Fig. 5,-E. It is evident that so long as the grid of tube 220 maintains the tube 220 in non-conducting condition the relay |80`cannot be operated in response to operations to marking of relay |40 during the signal series.

Upon the opening of the marking contact oi' relay |00, a potential of -50 volts on potentiometer |0| is removed from the oscillatory circuit or the impulse circuit which immediately commences its ilrst oscillation in the positive direction without any interfering transients. When the oscillationfhas increased to a predetermined positive potential applied to thegrid of vacuum tube 240, the cathode-anode path in this tube becomes conducting and remains conducting until the oscillation again decreases below this value, as shown in Fig. 5-F. Thus, a slowly rising and quickly falling current pulse iiows through the |00 to operate lback to marking, Abut leaving relays |10 andi in their spacing positions.. A v

Condenser III is now disconnected fromthe reference-,potential and connected tothe constant currentv conductor 5a showninFig.'5-J.

`Condenser .|5| is disconnected from the constant current conductor 5201:, having been` fior chargingI as l charged to a potential proportional to the duration of the preceding chargingtime. Thus, in the case of transition I arriving at the standard transition time the charge on condenser |5| would be such that its voltage would be equal tothe reference potential at the-marking. contact of relay |50; however, since transition Iv arrives early, thevoltage on condenser |5| isv less negative than the reference potential, as shownin Fig. 5-I, and at the instant -themarking contact oi' relay |50 closes fan equalizing current will ow in circuits that will now 'be traced.`

lOne ot these circuits includes the peak-voltmeter and may be traced from .condenser |5| detectortube and the primary of transformer the strong positive impulse in the secondary winding of transformer 14| may be oriented to occur exactly at the proper instant in relation to the center of each standard pulse period, as determined by the operations of relays |50 and |80 on distortion-free signals.

With the positive voltage from transformer 24| applied to the grid of tube 250 at the proper instant, this tube is fired and thereby connects the grounded inductance 255 over conductor 253,`

spacing contact of relay |10, constant current conductor 33611 and spacing contact of relay |50 to the unit condenser |5|, the charging of which thus will be discontinued, as shown in Fig. 5 1. Condenser |5|, which now is included in the oscillatory circuit with inductance 255, quickly discharges to zero potential, at which time the zero potential is applied over conductor 336a to the grid of gas-illlcd tube 260, ring this tube. 'I'he tube 250, in reducing the potential applied to one side of the charged condenser 255 impresses a negative potential on the plate of tube 250, thereby extinguishing this ytube and thereby also permitting the condenser |5| to again charge at a constant rate as shown in Fig. 5-I. Tube 250 through marking contacto! relay |50, resistance |52 and condenser |50 in multiple, conductor 0 42a, resistance 34|, primary of transformer/344, conductor 342 to the reference potential at point 340. Thus, a current impulse will be induced in the secondary winding oi.' transformer 344 through one of the rectiilers 25| and 252 and condenser'35l to ground. 'I'he consequent potential on condenser is applied to the tube 350 which thus sets up an unbalanced condition in the bridge circuit for the peak-voltmeter 2li! which ythus gives a deflection proportional to the currentstrength in transformer 344 and, therefore, proportional to the displacement oz. the marking transition I from the standard transition instant I.

Thev equalizing lcurrent from condenser |5| may be traced` over another circuit from marking con-fV tact of relay over break contact -b of relay I0, through the bias-meter |90, resistance |93,

break contact c of relay H8, condenser |53 anrj-l is subsequently extinguished by the oscillation "operated back to marking, causing relays |50 and resistance |62 in multiple and break contact el of relay ||8 to the referencepotential on the conductor 342e. The bias-meter |90, being highly damped by the large shunt condenser |02, will give integrated deflections for ia succession of equalizing impulses proportional to the average bias of the impulses being measured. This meter may be calibrated by adjustment of resistance |9i. In 4the present instant .the meter will be deflected to indicate the marking bias of transition I.

In the meantime, the impulse circuit is passing through its second cycle and the detector tube 240 is passing through its second conducting period atthe vend of which, and shortly before lthecenter- .of Athe second dot period, a positive impulse is applied from transformer' 24| totube 250'. '-As this tube becomes conducting, the discharging oscillatory circuit including inductance marking contact of relay |50 to condenser IBI, which now quickly discharges, as shown in Fig.

5-J.v When the potential of conductor 338e reaches zero by the discharge of condenser |5| through inductance 256, the gas tube 250 is again fired, thereby quickly extinguishing the gas tube 250 and subsequently extinguishingdtself by the discharging of condenser 255. When tube 250 becomes non-conducting, condenser ISI again charges up from the constant current conductor 6a until the arrival 'of the spacing transition 1I.

When transitionII arrlvearelays |50 and |50 operate to spacing and condenser now is connected for charging from the constant current conductor #25a and the charging of condenser |4| is discontinued. Since transition II arrived late with respect to the standard transition instant 2, the charge on condenser |5| is such that its potential is more negative than the reference potential. Therefore, when the marking contact of relay |00 closes, an equalizing current will flow' fore, because the transition II was displaced with respect to the transition instant 2 about as much as was transition I with respect to transition instant I.

At the same time bias-meter |00 is affected by the equalizing current through resistance |52 and condenser |53 in multiple over a circuit from the right side of resistance |52, break contact c of relay Il, through resistance |00, meter |00, break contact b of relay H0 through resistance |52 and condenser |50 in multiple to the left side of resistance |02, which is connected to the reference potential. The bias-meter will now be aifected in the same direction as for the previous reading for transition I, thus indicating marking bias.

The impulse circuit now enters its third cycle and near the middle of the standard/dot period condenser 5| is again discharged by the dashing of tube 250 and connected to recharge from the constant current supply.

The charging now continues until the transltion III arrives. As shown in Fig. 5-B this transition arrives later with respect to the standard transition instant 3 than did transition 1I rival or the p" transition to marking. with this transition arriving on time, there will be no equalizing current.

During the reception of all these transitions the condensers 222 and 222e have been slowly dis-r With the stop transition arriving on time relay will have operated to marking at the time tube 220 is conditioned for conduction. With plus battery applied from the marking contact of relay |40 to the plate circuit for tube 220, current flows through the tube and the upper 'winding of relay |00 and operates this relay to marking, thereby disconnecting the negative potential at potentiometer |0| from point 220 so that condensers 222 and 22-2a will be quickly charged from battery over resistance 22| and through the rectifier tube 225 to be ready for the next start" transition, see Figs. 5 0, D and E.

At this time relay |10 is operated to marking by its lower biasing winding, thereby again placing the stop-compensating voltage from conductor 022 upon the constant current conductor 338e and, therefore, uDOn condenser |0|. 'I'he constant current from tube 030 now owing through the potentiometer 020, the stop-compensating voltage applied to condenser |5| will be equal to the reference potential. Condenser |5| remains 40 connected to the reference potential.

with respect to transition instant V2. Therefore,

when relays |40, and |00 operate to marking,

a greater equalizing current flows from condenser III, giving a higher reading onthe peak-.voltmeter 310 and deflecting the bias-meter |90 towardsthe side indicating spacing bias.

Condenser |5| now charges up to the reference potential and near the center of the dot period it is discharged and then again charges at a constant rate.

With no transition IV, relays |40, |50 and |50 remain in marking position and condenser 5| continues to charge until the middle of the next dot period when it is again discharged and commences to recharge at a constant rate'.

. Transition V arrives on time so that, when re-` lays |40, |50 and |00 operate to spacing, the condenser |5| will be charged exactly to the reference potential and no equalizing current will flow.

Due to the rectifying action of the tubes 35| and 352 the charge on condenser 35| cannot be reduced when the equalizing current for a given transition is less than for a previous transition. Thus upon comparison of the transition V, the peak-voltmeter will continue to show the deflection for transition III; The bias-meter |00, having received no impulse since transition III, tends toward a zero-bias reading. Condenser l5| now charges, discharges and recharges until the ar- The operation of relay |00 to marking also placed a negative potential of 50 volts from potentiometer |0I upon the oscillating circuit for tube 230, and, due to the presence of the damping resistances 235 and 234, the oscillations are quickly restored to the steady state within the duration of the stop impulse, even allowing for a late arrival of the stop transition. With no impulses from the impulse circuit, the gas-filled tubes 250 and 250 thereafter remain dormant. The biasmeter tends to subside toward a zero reading and the peak-voltmeter 310 maintains its peak reading. The entire test circuit is, therefore, restored to normal awaiting the arrival of the next When the stop transition arrives early or late, there will, of course, be an equalizing current between condenser |5| and the reference potential, which may affect both meters.

If the "stop" transition should arrive so late that the charge on condensers 222 has already reached the point where the grid potential of tube 220 has conditioned this tube for conduction, the character timing circuit would await the operation to marking of relay |40 in response to the "stop transition, since tube 220 cannot conduct with negative potential applied to its plate from relay |40. Upon arrival of the stop transition the entire circuit would be restored to stop condition, as described above. There is thusno danger of the character timing circuit restoring the system to stop condition before the stop transition arrives, and it is insured that even a large displacement of the stop" transition will be accurately indicated on the meters.

When the "stop" transition is greatly delayed, say with a forty per cent spacing bias, a special condition arises. The operations of the system under this assumed condition are indicated in Fig. by dotted lines in the stop interval of the different diagrams. All the operations will progress as before, and the heavy equalizing current fromcondenser Iii due to the delayed "stop" transition will ailect the .meters I 00 and 810 as before. However, as the oscillatory circuit is returned to its steady-state condition by the operation of relay |00 to marking, an impulse may be produced in transformer 24|. Gas tube 200, which is now connected to conductor Ina only through resistance 212, since relay is operated to marking, will remain extinguished, resistance 212 being of such a high value thata discharge cannot be maintained. The condenser lil, being connected to the stop-compensating potential remains charged. Thus, there will be no effect upon the meter readings after the stop transition has been compared. y

From the description of the operation, presented above, it will be apparent that the tube 200 is made conducting for six short intervals, as shown in Fig. 5K, during which the unit condensers ISI or i8| are completely discharged.

These conducting periods thus establish uniformly distributed reference instants which may be oriented to occur exactly at the center of each standard dot period. By charging one or the other condenser with the constant current after each reference instant, the exact arrival time of the next transition may thus be determined quantitatively by means of the two meters. 1t will be noted that the bias-meter m during the operations incident to a signal series, as' just described. has given an average reading of the bias in both marking and spacing direction relative to the arrival time of the ,starttransition. It is, however, usually desirable to make separate readings for the average displacement of the mark-to-space transitions and the space-to-mark transitions relative to the arrival time of the start transition. For this purpose condenser ISI, the'charge of which is controlled by the space-to-mark transitions, is connected to the bias-meter in the marking position of relay through a normally closed key M-S. When this key is opened the charges of this condenser are prevented from affecting the meter reading which thus indicates the average displacement with respect to the start" transition of all the mark-to-` space transitions. Similarly, condenser |0| is connected over the spacing contact on relay |00 through a normally closed key S---M to the other side of the meter circuit so that, when this key is opened, the charges on this condenser will not affect the meter reading, which thus will4 indicate the average displacement of all space-to-mark transitions relative to the start transition.

This is the bias of the signals, provided no distortion other than bias is present.

Alternative modes of operation *circuit for condensers 222.

.circuit for resistance 223e which thus fis inserted or impulse generator 208, producing 120 impulses per minute, over key 201, conductor H3, make contacts c of relay |04 to the lamp |06, which will flicker at a fast rate, thereby indicating at the test set appearances that the circuit 'is adjusted to an abnormal condition. This signal is distinctly diierent from the demand signal of 60 impulses per minute impressed at certain times upon this lamp'through the interconnecting circuit, referred to above.

As already stated, the circuit is normally ar- -ranged for an operating speed of 23 cycles per second. Provision is also made for operating the set at other speeds. For an operating speed of 28 cycles per second, key |01 is operated, thereby closing an operating circuit over make contact d on relay |04, conductor H2, through a plurality of relays H6, 20|, 202 and 203, thereby operating these relays.

The operation of relay i|6 replaces the condensers IBI and |8| by condensers |5|a and lila, which are of smaller capacity to receive a standard charge corresponding to the same reference potential with the same constant current as before in a shorter time, corresponding to the speed of about 28 cycles per second, or 'l5 words per minute with the 71/2 unit code and 66 words per minute with the 81/2 unit code. For a given time displacement of a signal transition relative to the corresponding standard transition instant, the difference between the condenser potential and the reference potential will be greater for these smaller condensers than before so that the equalizing current will be greater. The greater meter deflections will thus indicate a greater distortion in per cent of the pulse period, which is proper. Since the change in the size of the test condensers is inversely proportional to the change of speed, it is not necessary to recalibrate meters and 310.

'I'he operation of relay 20| disconnects the cndenser 222a from the character timing circuit so that the time constant of this circuit will be changed to agree with the shorter interval between the start and stop transitions at the higher speed. The character timing circuit otherwise operates as before.

'I'he operation of relay 202 changes the tuning of the impulse circuit by replacing adjustable con,- denser 232 by adjustable condenser 232a over contacts d.y The decrement potentiometer 233 is also replaced by potentiometer 233a over contacts b and c. Thus, the circuitfor the tube 230 oscillates with a frequency corresponding tothe standard frequency of about 28 cycles' per second, and the decrement may be adjusted to maintain the amplitude of oscillation substantially constant during a complete pulse series representing a character. The damping resistance in the start circuit |82 is also changed at contacts a of relay 202 by the replacement of the resistance 234 by higher resistance 224a, to insure that the oscillator circuit, at the higher frequency, is returned .242a and 24M. The impulses through transformer 24| may thus be oriented within the shorter cycle `without disturbance of the orientation for the normal speed of 23 cycles per second.

In all other respects, the circuit is arranged the same and operates in the same manner as described above.

When it is desired to measure signals transmitted at the speed of 15 cycles per second corresponding to 40 words per minute with the 7 unit code, the key |01 must be operated thereby placing the circuit in the condition just described for the high speed. In addition, it is necessary to operate key 205, which adds relays |I1 and 204 to the relay-operating circuit ||2 from key |01.

The operation of relay connects the larger unit condensers |5|b and |6|b into the comparison circuit replacing the other condensers.

'I'he operation of relay 204 by its contact a, connects the condenser 222b in multiple to condenser 222 over a make contact on relay 20|. The character timing circuit is thereby slowed up t0 allow for the longer interval between the start and stop transitions. Relay 204, by its contact b, changes the frequency of the impulse circuit by adding the fixed condenser 2321 to the adjustable condenser 232a, thereby slowing up the frequency of the oscillator tube 230 to correspond to the standard dot frequency of about cycles per second. The start circuit |02 from relay |00 for the oscillating circuit is changed by resistance 23441 being short-circuited by contacts c of relay 204. It will, of course, be necessary` to readjust condenser 232a, potentiometer 233:1 and the orienting potentiometers 242a and 243a for recalibration of the set. Otherwise, `the circuit remains the same and operates as already described.

The operation of key 205 now places a warning signal 200 on the lamp |08 over make contact of relay conductor |I3a, key 201, conductor ||3 and make contacts c of relay |04 to the lamp 06. The signal produced by the signal generator 20B may be characteristically'diilerent from any other signals impressed upon lamp |06, but is preferably the same as that generated by the device 208 connected to the lamp by key 201.

For operation of the test set with a commercial circuit using start-stop signals of the polar type, it is necessary to operate the key |09. This key operates relay thereby disconnecting the biasing winding of relay so that this relay will be operated to marking or spacing in response to opposite directions of current in the line circuit through its upper winding. Otherwise, the test circuit operates as already described and may be set for the different speeds as already described.

A reset key |I0 is provided for discharging the condenser 36| in the peak-voltmeter circuit through the operation of relay 30|. This is for the purpose of dissipating the charge on condenser 36| and returning the meter 310 to zero reading before commencingeach observation.

The set may be calibrated for any particular type of service by connecting it through the jack |03 to suitable circuits producing standard undistorted signals of the kind to be measured.

Certain features included in the system described above and illustrated in the drawings are being claimed in a copending application for pat- `ent Serial No. 432,822, i'lled on even date herewith by J. R. Wilkerson, which has issued as ent No. 2,342,318 on February 22, 1944-.

What is claimed is:

1. An oscillation generator circuit for producing a predetermined series of voltage oscillations of substantially uniform waveshape which comprises a thermionic discharge vacuum tube having grid, anode and heated cathode, an oscillatory circuit including a capacitance and an inductance in multiple connection, a source of direct current connected in anon-reactive circuit between said anode and one side of said oscillatory circuit, said cathode being connected in a non-reactive circuit to the mid-point of said inductance, a non-reactive potentiometer connected from said mid-point to the other side of said oscillatory circuit and having an adjustable point connected to said grid for control of the amplitude of oscillations, a direct current source connected to place a biasing potential on said otherside of said oscillatory circuit to prevent Patoscillations, and timed contact means adapted for disconnectingsaid biasing source to start oscillations and for again connecting said biasing source to stop oscillations after a. predetermined number of cycles, said biasing potential being adapted to establish an initial energy condition at the starting instant in said generator circuit for immediate starting of the nrst cycle of 30 harmonic oscillation substantially Without extraneous transients. and saidpotentiometer being adjustable to effect a, substantially constant amplitude of the series of 'oscillations between the starting and stopping by said contact means.

2. A start-'stop telegraph signal testing system comprising a receiving and measuring circuit which includes a test condenser adapted to be charged during alternate pulses of a signal and to be substantially instantaneously discharged at a prexed reference instant of time at the beginning of each period of charging, a meter circuit adapted for measuring the charge on said condenser between said periods of charging, a source of constant current for charging said condenser, and polar relay means responsive to transitions between pulses of a signal for connecting said test condenser for said alternate charging and measuring, said testing system further comprising circuit means for said instantaneous discharging of said test condenser which includes a thermionic discharge vacuum tube with a control grid, an oscillatory timing circuit connected to said grid to cause flow of space current intermittently through said tube to establish said reference instants, and normally non-conducting condenser discharging means connected to said test condenser and responsive to said ow to intermittently become conducting for discharging of said test condensers.

3. A start-stop telegraph signal testing system comprising a receiving and measuring circuit which includes a test condenser adapted to have its initial standard charge changed at a predetermined rate during alternate pulses of a signal beginning at a preiixed reference instant of time within each of said alternate pulses and ending at the subsequent transition between successive pulses, a meter circuit adapted for measuring charges on said condenser at said subsequent transitions, a source of current for changing the charge on said condenser at said predetermined rate and polar relay means responsive to transitions between pulses for connecting said test condenser for said alternate change of charge and measuring, said testing system further comprising circuit means for establishing the said cludes normally non-conducting condenser charge control means connected to said'te'st condenser y. during said alternate pulses -and adapted to be intermittently rendered conducting for establishing said initial standard charge of said test concomprising ay receiving and measuring circuit which `includes a pair of test condensers adapted to be continuously charged alternately during corresponding alternate pulse periods of a signal andA to be momentarily discharged alternately at preiixed reference instants of time within their Period of charging, a meter circuit adapted for measuring charges on said condensers alternately between said periods of charging, a source of constant current for charging said condensers alternately, and polar relay means responsive to transitions between pulses of a signal for connecting said test condensers in alternations for said charging and measuring. said testing system further comprising circuit means for said momentary discharging of said test condensers which includes normally non-conducting condenser discharging means connected through said relay means to said test condensers during said charging periods and adapted to become intermittently conducting for quick discharging of said test condensers, an impulse circuit including an electronic vacuum tube and adapted to impress an impulse upon said discharging means on each occurrence of an electronic discharge within said tube to render said discharging means brieily conducting at each of said reference instants and also including capacitance means adjustable for timing of the discharges within said tube, and timing means including a second electronic vacuum tube and connected to stop operation of said impulse circuit on the occurrence of an electronic discharge within said second tube and also including capacitance means for timing the last said discharge to occur after a predetermined number of impulses.

5. A start-stop telegraph signal testing system comprising a receiving and measuring circuit which includes a pair of test condensers adapted to alternately have their initial standard charge changed at a predetermined rate during corresponding alternate pulses of a signal beginning at a prefixed reference instant of time within each pulse and ending at the subsequent transition between successive pulses, a meter circuit adapted for measuring charges on said condensers alternately at each transition between pulses, a source of current for changing the charge on said condensers at said predetermined rate, and polar relay means responsive to transitions between pulses for connecting said test condensers in alternations for said charging and measuring, said testing system further comprising normally non-conducting condenser charge control means connected by said relay means for quickly establishing said initial standard charge of said test condensers alternately once during each of said successive pulses, a tuned timing circuit connected to said charge control means and adapted to on said condensers alternately `between said pey y asv-chas standard charge of said test condenser which inrender said charge control means conductingfor said quick establishing of charge only at prefixed` intervals each equal to the standard duration oi' a signal pulse for establishing said reference instants, and tuned circuit means connected to said timing circuit and adapted to render said timingA circuit ineffective upon said charge-control means at the end of the interval required to send a signal series of pulses representing a character.

6. A start-stop telegraph signal testing system comprising a receiving and measuring circuit which includes a pair of test condensers adapted to be charged at a predetermined rate alternately during corresponding alternate pulse periods of a signal and to be substantially instantaneously discharged alternately at prexed reference instants of time within' their periods of charging, a meter circuit adapted for measuring charges rlods of charging, a source of current for charging said condensers at said predeterminedy rate,

and polar relay means responsive to transitions between pulses of a signal for connecting said test condensers in alternations for said charging and measuring, said testing system` further com'- prising normally non-conducting condenser discharging means adapted when rendered conduct. ing to quickly discharge either one of said test condensers connected by said relay means for charging at the moment, a character length timing circuit including a vacuum tube having grid, plate and filament and a slow-timing condenser connected to said grid and normally charged to render said tube conducting and having a diS- charge time approximately equal to the standard duration of a signal character, a start relay connected tol be responsive to an incoming start pulse t0 render said tube non-conducting and to establish a circuit for slowly discharging -said slowtiming condenser to thereby control the voltage on said grid for restoring the conducting condition of said tube and for restoring said start relay at the endof a signal character, a fasttiming circuit including a second vacuum tube having grid, plate and filament andan oscillatory circuit connected to the latter said grid and having a constant cycle duration corresponding to the standard duration of individual signal pulses, said fast-timing circuit being connected to said discharging means to render -saiddischarging means conducting at a predetermined time of each oscillation and being connected i'or starting and stopping of its oscillations by said start relay means. l

and including a direct current source, a nonreactive start circuit including a second direct current source and connected to said oscillatory circuit to establish a steady-state energy condi-v tion therein, and timed contact means for opening said start circuit to start the oscillations and for again closing said start circuit to stop the oscillations after a predetermined. number of connected to said inducy oscillations. said tube being adapted to operate with a constant amplification factor, and said control circuit including adjustable means for adjustment of the amplication of said tube to Amaintain a substantially constant amplitude of the oscillations between the starting and stopping by said contact means, in combination with a non-reactive output circuit for said oscillationl generator circuit including a second thermionic discharge means having grid, anode and heated cathode, the last said grid being connected in a non-reactive circuit to said oscillatory circuit for changing between conducting and non-conducting conditions of said second thermionic discharge means at a predetermined voltage of each cycle of oscillation, and the last said anode and cathode being connected in an impulse output circuit including a source of direct current.

8. .An oscillation generator circuit for producing a predetermined series of voltage oscillations of substantially uniform sine wave shape which comprises a thermionic discharge vacuum tube having grid, anode and heated cathode, a twowinding inductance having unit coeilicient of coupling, a capacitance connected to said inductance to form an oscillatory" circuit therewith, a non-reactive control circuit between one oscillatory winding of said inductance and the cathodegrid path in said tube, a non-reactive feedback.

. ment of the amplification of said tube to maintain a substantially constant amplitude of the oscillations between the starting and stopping by said contact means, in combination with a nonreactive output circuit for said oscillation generator circuit including a second thermionic discharge means having grid, anode and heated cathode, the last said grid being connected in a non-reactive circuit to said oscillatory circuit for changing between conducting and non-conducting conditions of said second thermionic discharge means at a predetermined voltage of each cycle of oscillation, the last said anode and cathode being connected in` an impulse output circuit including a positive and negative source of direct current and a potentiometer circuit connected across the last said source and having a iirst adjustable point connected for applying a low voltage between the last said grid and cathode and a second adjustable point connected for applying a higher voltage between the last said anode and cathode.

9. An oscillation generator circuit for producing a predetermined series of voltage oscillations of substantially uniform sine wave shape which comprises a thermionic discharge vacuum tube having grid, anode and heated cathode, a twowinding inductance having unit coeicient of coupling, a capacitance connected to said inductance to form an oscillatory circuit therewith, a nonreactive control circuit between one oscillatory winding of said inductance and the cathode-grid path in said tube, a non-reactive feedback cira,a7o,css l.

cuit between the' other winding of said inductance and the cathode-anode path in said tube and including a direct current source, a non-reactive start circuit including a second direct current source and connected to said oscillatory circuit to establish a steady-state energy condition therein, and timed contact means for opening said start circuit to start the oscillations and for again closing said start circuit to stop the oscillations after a predetermined number of oscillations, said tube being adapted to operate with a constant amplication factor, and said control circuit including adjustable means for adjustment of the amplication of said tube to maintain a substantially constant amplitude of the oscillations between the starting and stopping by said contact means, in combination with a non-reactive output circuit for said oscillation generator circuit including a second thermionic discharge means having grid, anode and heated cathode, the last said grid being connected ina non-reactive clr` cuit to said oscillatory circuit for changing between conducting and non-conducting conditions of said second thermionic discharge means at a predetermined voltage of each cycle of oscillation, the last said anode and cathode being connected in an impulse output circuit including a positive and negative source of direct current and v a potentiometer circuit connected across the last said source and having a iirst adjustable point connected for applying a low voltage between the last said grid and cathode and a second adjustable point connected for applying a higher voltage between the last said anodeand cathode, and said iirst and second adjustable points being adapted also to-be adjusted in unison for varying the grid-cathode voltage and at the same time maintaining the anode-cathode voltage constant for thereby varying the said voltage of each cycle at which said second discharge means changes from conducting to non-conducting condition.

10. An oscillation generator circuit for producing a predetermined Aseries of voltage oscillations of substantially uniform sine wave shape which comprises a thermionic discharge vacuum tube having grid, anode and heated cathode, a twowinding inductance having unit coefficient of coupling, a capacitance connected to said inductance to form an oscillatory circuit therewith, a non-reactive control circuit between one oscill latory winding of said inductance and the cathode-grid path in said tube. a non-reactive feedbackcircuit between the other winding of said inductance andthe cathode-anode path in said tube and including `a direct current source, a nonreactive start circuit including a second direct current source and connected to said oscillatory circuit to establish a steady-state energy condition therein, and timed contact means for opening said start circuit to start theoscillations and for again closing said start circuit to stop the oscillations after a predetermined number of oscillations, said tube being adapted to operate with a constant amplification factor, and said control circuit including adjustable means for adjustment of the amplification of said tube to maintain a substantially constant amplitude of the oscillations between the starting and stopping by 'said contact means, in combination with an impulse producing circuit connected thereto, said impulse producing circuit including a transformer and a second thermionic discharge means having grid, anode and heated cathode, the last said grid being connected in a non-reactive circuit to said oscillatory circuit for quick changing from conond discharge means at a predetermined descending voltage of each cycle of oscillations and for v a`,s7o,ess ducting to non-conducting condition of-'said seclrestoring the conducting condition at an ascending voltage of each cycle, the last said anode and cathode being connected in an impulse output circuit including a winding of said transformer and a potentiometer circuit in series connection and a positive and negative source ot direct curf ondary impulse in response to the quick cessation of current in said winding at each of said change 'able times of occurrence.

ll. A start-stop telegraph signal testing system comprising a receiving and' measuring circuit which includes a pair of testoondensers adapted to be continuously charged alternately during corresponding alternate pulse periods of a signal and to be substantially instantaneously discharged alternately at prefixed reference instants of time within their respective periods of charging, a meter circuit adapted for measuring charges on said condensers, alternately between said periods of charging, a source of constant charging current, and polar relay means responsive to transitions between pulses of a signal for connecting said test condensers in alternations for said charging and measuring, said testing system further comprising circuit means for said instantaneous discharging of said test condensers which includesan oscillation generator circuit for producing voltage oscillations of a substantially vsinusoidal wave shape which comprises a thermionlc discharge vacuum tube having grid, anode and heated cathode, a two-winding lnductance having unity coeillcient of coupling, a capacitance connected-to said lnductance to form an oscillatory circuit therewith, a non-reactive control circuit between one oscillatory winding of said inductance and the cathode-grid path in said tube, a non-reactive feedback circuit between the other winding of said lnductance and the cathodeanode path in said tube and including a direct current source, and a substantially non-reactive output circuit connected across said oscillatory circuit, said non-reactive output circuit including a second thermionic discharge means mounted in a vacuum tube'and having grid, anode and heated cathode, the last said grid being connected in a non-reactive circuit to said oscillatory circuit for changing between conducting and non-M conducting conditions of said second thermionic discharge means at a predetermined voltage of each cycle of oscillation, the last said anode and cathode being connected in an impulse output circuit including a source of direct current, said circuit means further vincluding normally nonconducting condenser discharging means adapted to be connected by said relay means to said test condensers during said period of charging and connected to be intermittently rendered conducting for discharging of said test condensers in response to the impulses in said impulse output circuit, a direct current biasing source connected to said oscillatory circuit to establish a steadystate snersyconaiuon in ma oscillatory maar,

and timed relay means connected` to be responsive to the first operation of saidpolar relay means for disconnecting said biasing source /to start the.' oscillations and timed -for again connecting said biasing source to stop the oscillations i p after a predetermined number of cycles, the oscillating frequency o! said generator circuit being equal to twice the standard pulse frequency'of the telegraph signals tobe tested.

12. An oscillator for producing a seriesof oscil-- lations of 'essentially sinusoidal wave form and of a single frequency which comprises an oscillation frequency establishing. circuit comprising capacitative and inductive elements, a repeating electron discharge device coupled to said circuit to sustain oscillations therein in the oscillatory condition, an energy source for said device,

-means for stopping and maintaining'said oscillator in-a non-oscillating condition, said means including means for storing the total energy inci-y dent to va given instant of steady-state oscillations in one or both of said capacitative and inductive elements, and means connected to said circuit for initiating a train of oscillations, whereby during the oscillating condition all wave cycles including the rst are of equal amplitude and periodicity.

' 13. An oscillation generator comprising a source of 'electrical current andv potential, a space discharge tube having an input circuit and an output circuit and an oscillation frequency determining circuit including inductance and capacity coupled to the input circuit and the output circuit; a control element of said tube being in the input circuit, means including a switch operable to one position to apply an oscillation preventing potential to said control element and to cause to flow in said lnductance a current from said source and simultaneously to supply to said capacity a charge such as would simultaneously exist in said frequency determining circuit at a given insta-nt during sustained steady-state oscillation, and said means being operable incident to another position of said switch to remove such oscillation preventing potential whereby the initial half cycle of oscillation is identical in amplicillations of essentially sinusoidal wave Iform and of a single frequency which comprises an oscillation' frequency establishing circuit including at least one capacitative and at least one inductive element, a repeating electron` discharge device coupled to said circuit to sustain oscillations therein in the oscillatory condition, an energy source for said device, means for stopping and maintaining said oscillator in a non-oscillating condition, said means including means for storing the total energy incident to a given instant of steady-state oscillations in said reactive elements, and means connected to said source for initiating a train of oscillations in whchduring the oscillation condition all Wave cycles including the iirst are of equal amplitude and periodicity, in combination with a non-reactive wave energy utilizing circuit coupled to said oscillator.

15. An oscillation generator ,comprising a source of electrical current and potential, a space discharge tube having an input circuit and an output circuit and an oscillation frequency determining circuit including lnductance and capacity coupled tothe input circuit and the output circuit, a control element of said tube being in the input circuit, and switch means operable to one position to apply an oscillation preventing potential to said control element and to cause to iiow in said inductance a current from said source and simultaneously to supply to said capacity a charge such as would simultaneously exist in said frequency determining circuit at a given instant during sustained steacbr-state voscillation, said switch means being operable to another position to remove such oscillation preventing potential whereby the initial half cycle of oscillation is identical in amplitude to subsequent half cycles in the same direction, in combination with a non-reactive potential control circuit coupled to, said oscillation generator to utilize the sinusoidal wave form produced thereby by withdrawing but negligible oscillatory energy therefrom. Y

16. An oscillation generator for producing a plurality of series of single frequency oscillations substantially of sinusoidal wave shape, successive sexies being separated by stop intervals without oscillations, comprising a frequency determining circuit comprising capacity and inductance ele-` ments, said circuit being intercoupled in feedback relation to an amplifying discharge device, start circuit means including a source of energy, and circuit control means conditionable to one condition for connecting said source to said frequency determining circuit to initiate each of said stop intervals and during each stop interval to admit substantially the total energy of oscillation to said frequency determining circuit for storing therein and questionable to another condition to' initiate each of said series of oscillations and release said stored energy for oscillation by disconnecting said source from said frequency determining circuit, in which said circuit control meansris conditionable selectively to said conditions .by incoming signals.

17. An oscillation generator for producing a plurality of series of single frequency oscillations substantially of sinusoidal wave shape, successive series being separated by oscillationless stop intervals, comprising a frequency determining circuit including at least one capacity and at least one inductance element, said circuit being intercoupled in feedback relation to an amplifying discharge device, start circuit means including a source of energy and circuit control means conditionable to one, condition for connecting said source to said frequency determining circuit to initiate each of said stop intervals and admit substantially the total energy of oscillation to said frequency determining circuit for storing 

